Coordinated Universal Time (UTC), official international time, will be adjusted by the addition of one leap second at the end of June 30, 2015. The leap second will be added at 23:59:59 UTC on June 30, 2015.
The upcoming leap second scheduled for June 30, 2015 will be the first leap second in many years to occur during a time of significant normal business activity. For example, local time for leap second implementation will 7:59:59 PM Eastern Daylight Time or 4:59:59 PM Pacific Daylight Time on Tuesday, June 30, 2015. Leap second in 2005 and 2008 occurred at the end of December 31. The most recent previous leap second in 2012 occurred at the end of Saturday, June 30, 2012.
Because many systems rely on precision timing synchronized to UTC, several U.S. government agencies including NIST are providing "Best Practices for Leap Second Event Occurring on June 30, 2015" to help interested people most effectively respond to the upcoming leap second.
UTC is determined by atomic clocks across the world (including NIST), and is adjusted at irregular intervals to keep UTC within one second of mean solar time. Mean solar time varies from UTC because of irregular changes in the rotation rate of the Earth. The June 30, 2015 leap second will be the 26th leap second in UTC since this system of timekeeping was implemented in 1972.